Synthetic polyelectrolytes, dissociation

A Comparison of the Dissociation Kinetics of Rare Earth Element Complexes with Synthetic Polyelectrolytes and Humic Acid... 

Since ionic polysaccharides are stiffer than most synthetic polyelectrolytes, they were thought to be appropriate experimental models to test theories based on cylindrical symmetry. Systematic determinations of co interion and coion diffusion coefficioits and activity coefficients have been made in the presence of ionic polysaccharides these measurements pertain to the interaction of small ions in the ionic atmosphere with the polyelectrolyte. Here, resiilts are presented for estimating the fraction of ions dissociated from the ionic polysaccharide in simple salts solutions. 

A synthetic ion-selective (ion-exchange) membrane is a dense, nonporous, mechanically stable polymer film about 0.01 —0.04 cm thick. By nonporosity we mean the absence of pores (possibly very tortuous transmembrane channels) with a typical radius above 5 — KM (10-8 cm). Structurally the membrane material is a cross-linked polyelectrolyte. This latter is a polymer containing chemical groups that while in contact with an aqueous solvent are capable of dissociation into charges which remain fixed to the polymer core and counterions which are free to move in the solution. 

Synthetic polymers are widely applied to modify the surface properties of materials, and their adsorption mechanism is very different from small ions or molecules discussed in previous sections. Moreover, special methods are applied to study polymer adsorption, thus, polymer adsorption became a separate branch of colloid chemistry. Polymers that carry ionizable groups are referred to as polyelectrolytes. Their adsorption behavior is more sensitive to surface charging than adsorption of neutral polymers. Polyelectrolytes are strong or weak electrolytes, and the dissociation degree of weak polyelectrolytes is a function of the pH. The small counterions form a diffuse layer similar to that formed around a micelle of ionic surfactant. 

Ions, defined as particles that carry electrical charges, exist in condensed phases (solids and liquids) as electrically neutral combinations of cations and anions electrolytes. The ions may be bound or relatively free to migrate. Ions may be monatomic, such as K+or Cr, they may consist of a few atoms, such as ammonium, NH4+, or sulfate, S04 , or even considerably more than a few, such as nitrobenzoate, 02NC6H4C02, or tetrapropylammonium, (C3H7)4N+. They may even consist of very many atoms and may carry many dispersed charges and are then referred to as poly-ions, constituting the dissociated part of polyelectrolytes. Some biological moieties, such as polypeptides, proteins, and nucleic acids, as well as suitable synthetic molecules are examples of polyelectrolytes. 

All these polymers are electrically neutral. If chains are built up of monomers that contain an ionizable group, i.e., a group that can dissociate into a chain-fixed kation or anion and a mobile counter-ion bearing the opposite charge, a polyelectrolyte is obtained. Table 1.2 collects a few typical examples. The first three compounds are synthetic polymers, the other two samples are biopolymers cellulose and starch in the form of derivatives which include ionizable substitutes. 

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